1. Field of the Invention
The present invention relates to a magnetooptical recording medium which may be used for a magnetooptical memory, magnetic recording, and display element, and which can be read out by magnetooptical effects, such as the magnetic Kerr effect and Faraday effect, and, more particularly, to a magnetic thin film recording medium having improved corrosion resistance.
2. Description of the Prior Art
Heretofore, there have been known various magnetooptical recording mediums, for example, polycrystalline thin films such as MnBi, MnCuBi and the like, amorphous thin films such as GdCo, GdFe, TbFe, DyFe, GdTbFe, TbDyFe and the like, and single crystalline thin films such as G/IG and the like. Such films are disclosed in U.S. Pat. Nos. 4,126,494, issued Nov. 21, 1978, and 4,152,486, issued May 1, 1979.
Among these thin films, the amorphous thin films have been recently regarded as excellent magnetooptical recording mediums, since such thin films of a large area can be produced at about room temperature, signals can be written with a small light-thermal energy at a good writing efficiency, and the written signals can be read out at a good S/N ratio at a high read-out efficiency.
Among the above-mentioned thin films, GdTbFe has a large angle of Kerr rotation and a Curie temperature of about 150.degree. C., so that it is regarded as suitable for a magnetooptical recording medium. The angle of Kerr rotation is defined as stated below. A linearly polarized light incident on a magnetooptical thin film is reflected as an elliptically polarized light. The angle of Kerr rotation is the angle formed by the polarized plane of the incident light and the plane formed with and containing major axes of the resulting elliptical polarized light.
The present inventors have conducted research to improve the angle of Kerr rotation and have found that GdTbFeCo, as disclosed in parent application Ser. No. 491,300, the disclosure of said application Ser. No. 491,300 being expressly incorporated herein by reference, has a sufficient large angle of Kerr rotation and is a satisfactory magnetooptical recording medium capable of being read out with a good S/N ratio.
However, amorphous magnetic materials such as GdTbFe, GdTbFeCo and the like, are known to have a poor corrison resistance. That is, their magnetic characteristics deteriorate in contact with air and moisture, and are finally completely oxidized to become transparent. For this reason, there have been proposed heretofore disk-type recording mediums having a protection layer on the magnetic film and a "gas-sandwiched type" having an inert gas sealed in its internal space, but such devices do not have sufficient corrosion resistance.